The present invention relates to a treatment for stainless steel to passivate a surface of the steel by removing adsorbed and absorbed moisture and by enhancing corrosion resistance to corrosive materials. More particularly, the present invention relates to such a surface passivation treatment wherein the surface to be treated is flushed with a dry chemically non-reactive gaseous fluid containing essentially no oxygen while the steel is baked for a predetermined time and temperature and thereafter cooled.
In ultra-high purity gas distribution systems that contain piping, valves, chambers and etc., it is important that the system itself does not contaminate the gas to be distributed by adding contaminants such as moisture and particulate matter to the gas. With respect to moisture, ultra-high purity gas distribution systems are generally flushed with an inert gas prior to use in order to outgas moisture and therefore prevent moisture contamination during subsequent operation of the system. In order to prevent possible particulate contamination due to corrosion, the components of ultra-high purity gas distribution systems are commonly fabricated from stainless steel. In the prior art it is known that stainless steel is resistant to corrosion because it possesses a surface enriched in chromium oxide. Generally speaking, the higher the content of chromium in stainless steel, the more resistant the steel is to the effects of corrosion. However, when corrosive gases such as hydrogen chloride or silane are to be distributed, even stainless steel components can react with the gasses to add unacceptable amounts of contaminants to the gas to be distributed.
The corrosion of concern in the prior art concerns resistance to chloride attack by neutral pH, aqueous salt solutions rather than to corrosive gases. It is known that corrosion resistance to such chloride attack at the surface of a polished stainless steel component can be enhanced by baking the component in a high vacuum furnace to enrich the chromium oxide content of the surface of the component. For instance, Asami et al., "Changes in the Surface Compositions of Fe--Cr Alloys Caused by Heating in a High Vacuum", Corrosion Science, Vol. 18, 1978, pp. 125-137, discloses that when polished stainless steel is heated in a vacuum at a temperature of about 380.degree. C., enhanced chromium surface enrichment can be observed by x-ray photo-electron spectrographic techniques. Hultquist et al., "High Protective Films on Stainless Steels", Material Science and Engineering, Vol 42, 1980, pp. 199-206, discloses a method for enhancing the corrosion resistance of stainless steel in which the steel is baked at a temperature range of between about 277.0.degree. C. to about 477.degree. C. in a high vacuum furnace. Furthermore, Adams, "A Review of the Stainless Steel Surface", Journal of Vacuum Science Technology, Vol A1(1), 1983--pp. 12-18, discusses heating type 316 stainless steel in a temperature range of between about 250.degree. C. to about 500.degree. C. in partial pressures of oxygen of 5.times.10.sup.-7 Torr to about 10.sup.-5 Torr to produce chromium enrichment and enhanced corrosion resistance.
A central disadvantage of such prior art techniques, as discussed above, is that they all involve the use of high vacuum equipment which adds to the expense and complexity of the treatment. In any event, the prior art has not applied techniques that involve baking polished stainless steel under conditions of vacuum or low partial pressures of oxygen to chemically passivate the surface of stainless steel against corrosive gases such as hydrogen chloride gas and silane.
As will be discussed, the present invention provides a passivation treatment for stainless steel that is effective to provide resistance to surface chemical reactions between stainless steel and corrosive materials without the use of expensive vacuum equipment while reducing the degree to which the stainless steel will outgas moisture. An important added benefit is that even after the stainless steel has been exposed to moisture the treatment, the subsequent flushing time involved in reducing the moisture outgassing of the steel to very low levels is also reduced.